Fuel level responsive vent valves are well known for use in automobile fuel tanks, located in the tank to remain open when the fuel is below a certain level, and to close when the fuel reaches the valve. Two common applications of these valves are "rollover" valves, which respond to abnormal fuel levels or unusual vehicle angles to close a vapor outlet from the tank, and fuel shutoff or "fill control" valves which are positioned to close when the fuel tank reaches the "full" level during refueling.
Depending on known factors such as the valve's intended application, the size and shape of the fuel tank, and the desired "full" or "rollover" fuel level, the height of the valve in the fuel tank (or distance the valve is spaced from the top wall) will vary. For example, two different car models with two different fuel tanks might require the same valve mechanism, but located at different heights in the respective tank.
Additionally, fuel level responsive vent valves are now commonly supported from the top wall of the fuel tank by a job-specific cover or cap which can be attached to the top in a sealing fashion such as with a grommet or bayonet mount or by welding to the outside of the fuel tank wall. The top cover is typically preassembled to the main valve body, and the valve body is inserted through a hole in the top wall of the fuel tank until the top cover contacts the tank's upper surface where it is locked or welded in place.
The top cover and the main valve are often made from different types of plastics, for example relatively soft, weldable polyethylene (PE) for the top cover and relatively rigid, non-weldable nylon for the valve body. The interface between these different materials must be sealed to prevent vapor and fluid leaks, and must remain sealed throughout the fluctuating temperature and vapor pressure conditions of the fuel tank over the life of the valve. The different materials, however, often respond differently to the same environmental conditions in the tank, for example with the portions of the top cover in the tank tending to grow or "creep" in length. This can affect the integrity of the seal between these two main parts of the valve assembly.
It may also be necessary to attach the same type of valve body to different types of top covers with different manners of attachment to the fuel tank.
It is also desirable to make the sealing interface or connection between the non-weldable valve body and the weldable top cover as impact-resistant as possible to maintain the integrity of the sealing fit therebetween.